Revert "Upgrade protobuf-java to 3.4.0"
This reverts commit 7786dcc3da859d86757e13ca36ee46c083b088f1.
This should have been done after releasing a version of gitiles
with upgraded JGit that can be used in gerrit 2.15.
Change-Id: Icae01d5b392366381ca05c01aeb86e73f9f440af